How about something like this to obtain a visual solution:
bughistory[ic_List,iters_]:=NestList[{#[[1]]*
Exp[0.63((1 - #[[1]]/50)) - 0.068#[[2]]], #[[1]]*(1 -
Exp[-0.068*#[[2]]])} &, ic, iters];
ListPlot[bughistory[{25,10},200],PlotJoined->True]
"Mark Hunter" <mhunter at ecology.uga.edu> wrote in message
news:8lj8q0$8rr at smc.vnet.net...
> I'm trying to use mathematica to run insect population models. I'm new
> to the system, and wolfram support suggested that I post my question
> here.
>
> The models are simple predator-prey models. They are non-linear
> simultaneous difference equations. I've tried using RSolve, but it
> doesn't seem to handle the non-linearity. Is there a way of using a
> Do-Loop for the same kind of thing? A simple example of the model that
> I tried is:
>
> RSolve[{a[n + 1] == a[n]*Exp[0.63((1 - a[n]/50)) - 0.068b[n]],
> b[n + 1] = a[n]*(1 - Exp[-0.068*b[n]]), a[0] == 25,
> b[0] == 10}, {a[n], b[n]}, n]
>
> I want an output for variables a and n at time n, n+1, etc.
>
> I'd be most grateful for any advice that you can offer, and please don't
> underestimate my current level of ignorance.
>
> Many thanks,
>
> Mark Hunter
>